This invention relates to apparatus for protection of, for example, an electric light from flicker due to voltage drop when a servo motor starts and, more particularly, this invention is applicable to a servo motor driven sewing machine installed in a household where its power supply is limited.
It has been found that, to increase sewing efficiency, sewing machine operators used to pedal down the sewing machine foot switch so quickly to rotate the servo motor at high speed that the electric voltage at the sewing factory was suddenly dropped and caused what is called "flicker". To alleviate this problem, an apparatus for protection of flicker when a servo motor starts has been suggested as shown at FIG. 3.
Referring to FIG. 3, AC current source 1 is connected to rectifier 2 and resistances 3, 4. Other resistances 5, 6, which branch from the connection of resistances 3, 4 and a condenser 7 in parallel with resistance 6 are arranged as shown. These parts constitute a reference-voltage-setting circuit. Resistances 8, 9 and variable resistance 10 are arranged as shown and these parts constitute a comparative-voltage-setting circuit. A servo motor 11, collector and emitter of the transistor 12, and resistance 13 are arranged in series as shown. The resistance 13 is used for detecting electric current The connection of the resistances 5, 6 is input to the minus terminal of the first comparator 14.
The output from the variable resistance 10 is connected to the plus terminal of the first comparator 14. A feedback resistance 15 is connected to the first comparator 14. Resistances 16, 17 in series with electric source V.sub.cc constitute a electric-current-setting circuit. Voltage at the connection of the resistance 16 and the resistance 17 is connected to the plus terminal of the second comparator 18 and to the plus terminal of a third comparator 19. The emitter of the transistor 12 and the resistance 13 are connected to the minus terminal of the second comparator 18. The minus terminal of the third comparator 19 is connected to output of the first comparator 14.
The output from the comparator 19 is connected to a control circuit 20. The output from the control circuit 20 is connected to the base of the transistor 12. A pedal 21 is connected to the control circuit 20.
In the suggested arrangement, when an operator pushes pedal 21, since the output from the second comparator 18 is high, control current from the control circuit 20 flows to the base of the transistor 12. The circuit between the collector and the emitter closes, electric current flows to the servo motor 11, and the servo motor 11 starts rotation. At this moment, the electric current to the servo motor 11 flows to the resistance 13 which is used to detect the electric current.
To insure that this electric current is less than a set current level, for instance 20A, the voltage at the resistance 13 is connected to minus terminal of the second comparator 18 such that the electric current at the resistance 13 will be kept less than 20A. To do so, output from the second comparator 18 is maintained low.
On the other hand, when the electric current flows to the servo motor 11, output from the variable resistance 10 becomes low, so the plus side voltage of the servo motor 11 drops. Resultantly, the output from the first comparator 14 becomes low, the output from the third comparator 19 becomes high, and thus electric current to the servo motor 11 is stopped.
According to the aforementioned suggested arrangement, when the servo motor 11 starts and comparatively large electric current flows, the electric voltage drops and shuts out the electric current from the control circuit 20. Accordingly, the servo motor 11 acts intermittently, the motor, pulley, and belt vibrate simultaneously, and therefore the machine as a whole vibrates. When an operator pushes the pedal 21 strongly, the electric current to the servo motor 11 becomes low, its rotation speed is lowered, and the sewing efficiency is lowered.